Conventional vehicle brake systems of the general type under consideration include hybrid systems in which the application energy for actuating the vehicle brakes is provided in two ways. The brakes on at least a first axle of the vehicle are equipped with brake cylinders operated by pressurized fluid, and can be actuated by the brake cylinders so that the application energy is supplied via pressurized fluid. In contrast, the brakes on at least one other axle of the vehicle can be actuated exclusively electromechanically, and, so, the application energy of these brakes is supplied electromechanically.
Conventional electro-pneumatic or pneumatic brake systems typically provide the application energy for actuation of vehicle brakes in a single way. The brake cylinders can all be actuated electro-pneumatically or pneumatically. Pneumatic pressure can be modulated and, if necessary, a braking request signal can be generated by means of a brake pedal device. According to the braking request signal that may be present, pressure can also be modulated by means of a valve unit. A modulated pressure can be amplified by means of relay valves and supplied to the pneumatically operated brake cylinders for actuation of the brakes. For this purpose, conventional electro-pneumatic or pneumatic brake systems are typically equipped with two brake circuits operated by pressurized fluid, each of which can be supplied with compressed air from a compressed air reservoir tank. The compressed air reservoir tanks are in communication with the valve unit and the pneumatically operated brake cylinders via compressed air lines.
In these known service brake systems, a parking brake can typically be applied to at least one rear axle of the vehicle. For this purpose, the brake cylinders on the brakes of this rear axle are equipped with spring actuators. In the unpressurized condition of the spring-actuated brake cylinders, the parking brake is applied by means of the spring actuators. For driving operation, the parking brake must be released. For this purpose, compressed air can be admitted to the spring actuator part of the spring actuator brake cylinders by establishing communication with one of the compressed air reservoir tanks.
A disadvantage of these conventional electro-pneumatic or pneumatic brake systems is that they are equipped with pneumatically operated brake cylinders on all brakes. These pneumatically operated brake cylinders can be supplied with compressed air via two compressed air circuits. Thus, many expensive components need to be incorporated into such service brake systems. In addition, the complexity of the tubing is great. This leads to high installation costs and to increased susceptibility to faults due to possible leaks in the compressed air lines. Maintenance time and effort as well as down times resulting therefrom also contribute to high costs.
Hybrid systems are more cost-effective than conventional hydraulic or pneumatic brake systems. In particular, hybrid systems are equipped on at least one vehicle axle with comparatively inexpensive electromechanical brake actuating devices, without the need for comparatively more expensive brake cylinders operated by pressurized fluid. A hybrid brake system is described in JP 2002-067909 and includes hydraulically actuatable brake cylinders on a first group of wheels and electromechanical brake actuating devices on a second group of wheels. In this case, the wheels of the first group are typically wheels of a front axle, whereas the wheels of the second group are typically wheels of a rear axle.
Two mutually independent pressure circuits are used for the brakes on the left front wheels and for the brakes on the right front wheels. The hydraulic pressure that can be modulated by means of a brake pedal device can be amplified by means of valve units and supplied to hydraulically-operated brake cylinders.
The pressure modulated in the hydraulic brake circuits is measured. In addition, wheel rotational speeds as well as the torques of motors of electromechanical brake actuating devices are sensed by means of sensors. The electromechanical brake actuating devices are controlled using the sensor data. For this purpose, the measured value of the modulated hydraulic pressure of the right front vehicle wheel is used to control the electromechanical brake actuating device for the left rear wheel. At the same time, the modulated hydraulic pressure for the hydraulically actuatable brake of the left front wheel is used to control the electromechanical brake actuating device for the right rear wheel. This crosswise logical combination of front brakes and rear brakes prevents failure of all brakes on one side of the vehicle in the event of failure of one brake circuit.
According to JP 2002-067909, the electromechanically actuatable brakes are not actuated if the vehicle is moving at very low speed, if the vehicle is at a standstill or if a parking function of the vehicle is activated. As a result, electrical energy is conserved.
The known hybrid brake systems are all equipped with a combination of hydraulically actuatable brake cylinders for braking wheels on at least one (front) axle of the vehicle as well as with electromechanical brake actuating devices on at least one other axle (see e.g., the hybrid brake systems disclosed in EP 1352799 A2, U.S. Pat. No. 5,390,992, DE 19622017 A1 and EP 1785324 A1).
JP 2002-067909 discussed above further proposes that the hybrid brake system also be used for a vehicle train including a tractor and a trailer. This is largely non-problematic if the trailer is equipped with electromagnetically actuatable brake actuating devices. However, many trailers are equipped with brake cylinders operated by pressurized fluid. A tractor should therefore be capable of forming a vehicle train and braking it safely even with a trailer equipped with brake cylinders operated by pressurized fluid. Restriction to the use of trailers having brake actuating devices that are actuatable exclusively by electromechanical means is usually not desired—a tractor should be capable of being coupled with different trailers.
The use of a hydraulic brake circuit with a trailer has the disadvantage, however, that hydraulic fluid can escape while the trailer is being coupled or uncoupled. Escaped hydraulic fluid can pollute the environment and should therefore be collected and professionally disposed of. Moreover, any loss of hydraulic fluid must be replaced. Hydraulic fluid is already expensive, and the associated time and effort for maintenance can also be costly.
The known hybrid service brake systems, while more cost-effective than conventional service brake systems operated purely by pressurized fluid, nevertheless are less suitable for use in a vehicle to which it must be possible to couple a trailer in flexible manner.